Processing of ornamental stone



June.l4, 1938. 0, gg o A PROCESSING OF ORNAMENTAL STONE Filed Aug. 50, 1937 attorneys Patented June 14, 1938 UNITED STATES PATENT oFFLcs 4 Claims.

This invention relates to a method of processing ornamental stone in order to alter or improve its ornamental characteristics. More particularly, the invention is concerned with the treatment of marble having colored veins so as to change the color of those veins to provide a variety of color combinations, and also in certain cases to improve the appearance of marble having undesirable color characteristics.

Numerous objects and advantages of the invention will be apparent from the following specification when read in accordance with the accompanying drawing in which:-

Figure 1 is a vertical section through one form of apparatus suitable for processing ornamental stone according to this invention; and

Fig. 2 is a perspective view showing the arrangement of the stone slabs as they are placed in the process apparatus.

It is well known that the desirability of ornamental stones and particularly of marble is improved by the presence of colored veins running through the base structure. Numerous color combinations are found in nature, and the veining is frequently caused by the presence of different iron compounds, the common ones being limonite and hematite. The limonite is yellow and forms a contrast with a white or gray base color of the stone, but there is a demand for marble in which the colored veins are red, pink, or otherwise highly colored. I have found that it is possible by heat treatment in a furnace to change the appearance of marble slabs containing veins of limonite by subjecting the slabs to a temperature sufiicient to change the limonite to hematite without, however, impairing the structure of the marble. The resulting product then consists of a white or grayish body having rose or red veins running through it. In this way, a variety of color combinations may be had starting with the product of a single quarry. The processing can be carried out by relatively simple apparatus.

I have found that the chemical change from limonite to hematite can be caused to occur by heating the marble not exceeding 500 F. It is necessary to take adequate precautions to prevent the marble slabs from cracking during the heating and cooling process and also to prevent overheating of any part of the slab, such as would produce local deterioration of the slab. In order to explain more clearly how the process may be carried out, Fig. l of the drawing shows a muffle furnace comprising a body 3 of refractory material having a closed middle shell 4 within it and supported by bars5. The shell '4 contains shelves 6 and retainers I suitably arranged to receive a stack of marble slabs 8 with interposed-metal plates 9 of copper or other good heat conducting metal. Any suitable arrangement may be adopted for holding the slabs in assembled relation, the arrangement shown in Fig. 1 being only one manner in which this may be accomplished. The furnace 3 may contain any suitable heating means such as the gas burners II, and has the usual offtake flue [2.

As the invention is at present practiced, slabs of marble having a thickness varying from to 4" and of any desired size having dimensions even as high as x 12 are stacked up with interposed sheets of heavy copper 9, these sheets being of greater area than the slabs 8 so as to form projecting heat radiating fins. The stack of slabs is placed in the furnace as indicated in Fig. 1, with the slabs and. plates or sheets preferably on edge and held in closely assembled relation. The furnace is gradually heated up to a temperature not exceeding 500 F. The heating is carried on at a slow enough rate to prevent cracking of the marble slabs. The time required for heating may be as low as a fraction of a day, depending upon the thickness of the slabs and the total bulk of the stack which is under treatment.

As the temperature rises, the limonite veins I3 change from the yellow color of limonite to a rose or red color characteristic of hematite; The intensity of the coloration will depend upon the temperature employed and to some extent on the duration of treatment. Experiments with each particular kind of stone will readily demonstrate the time required to bring about any particular desired coloration. The slabs must be cooled slowly in order to prevent warping and cracking and, while the heating of the furnace may be carried on at a relatively rapid rate, it will usually require a longer time, perhaps a week in some cases, to cool the slabs to atmospheric temperature without detrimental effect. While the metal plates 9 are indicated as of copper, they may be of other metals having high heat conductivity and hence capable of delivering the heat evenly to the entire area of the slabs under treatment.

While it has been indicated in this description that the ornamental stone under treatment is marble, that being the purpose for which the process is primarily intended, the invention is not limited to this particular stone but may be applied to the treatment of slabs of stone containing veins or other inclusions capable of alterthe quarries of Missouri in the vicinity of Ste..

Genevieve, but the process is not limited to the treatment of stone from this region.

What is claimed is:

1. The method of treating stone in slab form which consists in assembling a plurality of stone slabs interleaved with plates of metal andheating them in a closed chamber to a temperature sufficient to change the color of the stone and then slowly cooling the slabs to prevent distor tion of the same.

2. The method of treating slabs of stone containing colored veins contrasting with the color of the base stone which consists in assembling slabs of stone interleaved with metal plates of greater cross sectional area than the slabs, heating them in a closed chamber to cause the veins to change color, and then slowly cooling the stone to prevent cracking or distortion.

3. The method of changing the color of ornamental stone which consists of heating a stack of stone slabs interleaved with metal plates until the stone undergoes a chemical change sufiicient to result in a color change, and then cooling the slabs to atmospheric temperature at a rate slow enough to prevent cracking.

4. The method of heat treating stone slabs which consists in interleaving the slabs with heat conducting plates and thus causing even heating and subsequent even cooling of. the slabs primarily by inducing flow of heat to and then from 20 the slabs through said plates.

JOHN O. COLONNA. 

